Originally, the head-up display (HUD) is the display system for a military fighter plane. With the HUD, the pilot is able to view the important information of the assignment without having to lower his/her head. In this way, the attention is not interrupted. Since the HUD is able to provide improved security, the HUD has been broadly applied in the market of vehicles in recent years.
HUDs can be divided into “reflective type” and “virtual image display” type. The “reflective type” HUD uses a low-transmissive film in a deep color to reflect an image projected by a projection unit. However, the image of the “reflective type” HUD can only be displayed on the windscreen panel, making the user feel tired visually due to alternately viewing at long and short distances when paying attention to the traffic and to the contents of the display. Also, the “reflective type” HUD is unable to display an image with a higher resolution. The “virtual image display” HUD solves the aforesaid issues. The “virtual image display” includes a projection unit emitting an image beam, a real image forming device receiving the image beam and forming an image, such as a transparent diffuser, and a virtual image forming unit that enlarges a real image into a virtual image. In the conventional art, an obscure glass is commonly used as the diffuser. However, the obscure glass is formed by applying a surface treatment process such as mechanical sand blasting, manual polishing, or corrosion with hydrofluoric acid to a flat glass. The obscure glass has a rough surface and an uneven structure. Thus, when the obscure glass is used as a diffuser in the projection apparatus, the image formed accordingly may contain irregular shades due to microstructures on the rough surface. Also, the coherence of laser may result in interferences on the surface of the diffuser, leading to generation of sparkle noises of laser. Thus, the image quality of the virtual image generated by the projection apparatus may be unfavorable.